1. Field of the Invention
The present invention relates to a communication apparatus capable of interfacing with fax and audio signals and relates to a processing method adopted by the apparatus.
2. Description of the Related Art
In recent years, the amount of data communication traffic has been displaying an increasing trend while the amount of audio communication traffic has been showing a decreasing trend among various kinds of traffic in a network. In a number of networks, the amount of data communication traffic already exceeds the amount of audio communication traffic. In the future, it can be said that the amount of data communication traffic will further display an increasing trend and a fraction occupied by audio communication will show a decreasing trend. Since real-time conditions are not required for data communication, however, the band is allowed to change dynamically. On the other hand, audio communication requires that real-time conditions be met. Thus, a fixed band must be allocated. During an audio communication, the allocated fixed band is completely occupied. Thus, audio communication always consuming a fixed band even in a mute state, and consumption of a fixed band in a mute state is not desirable if seen from a standpoint of network utilization efficiency. Therefore, it is necessary for a communication apparatus in a communication network to compress voices and sounds, to reduce the size of a band allocated to the voices and sounds and thus to increase a multiplexing rate of the voices and sounds due to the compression and the down sizing in order to raise a communication efficiency and, hence, reduce the communication cost. Since real-time conditions are not required for data communication as described above, on the other hand, a band allocated to data can be dynamically changed in a general communication of the data in accordance with factors such as the amount of data traffic and priority.
A fax communication can be normally carried out by allowing the fax communication to share a telephone network with telephone communications. Thus, when a method of compressing an audio band in a communication network is adopted, the compressed band is supposed to be usable for a fax communication. Since a coding & decompression algorithm adopted in an audio compression/decompression functional unit for compressing and decompressing human voices is set specially for human voices, however, the compression & decompression mechanism is not applicable to fax communication. Thus, a fax communication must be carried out without using the audio compression/decompression functional unit. In order to implement such fax communication, a fax termination function is required. To put it concretely, a fax termination unit of a communication apparatus and a fax terminal connected to the fax termination unit serves as a receiver and a transmitter respectively in a fax communication. That is to say, the fax terminal on the transmission side carries out a fax termination with the reception-side fax termination unit connected to the fax terminal. In such a fax communication, the fax termination unit of the communication apparatus actually relays the communication so that the fax terminal appears to have a fax communication with the fax termination unit to the bitter end.
A communication apparatus handling voices, sounds and data has a data-communication interface and an audio-line interface, which allow interfacing with both an audio line and a data communication line. Such a communication apparatus supports communications such as VTOA (Voice Over ATM), VOIP (Voice Over IP) and VOFR (Voice Over Frame Relay). The VTOA is communication through an ATM network by accommodation of an audio signal in an ATM cell. The VOIP is communication through an IP network by accommodation of an audio signal in an IP packet. The VOFR is communication through a frame-relay network by accommodation of an audio signal in a frame-relay signal. The audio-line interface must support voice communication and fax communication like the conventional audio line as described above. In order to reduce the audio band by as much as possible, however, an audio compression/decompression functional unit in an audio interface unit employed in an audio-line processing unit decreases the size of the audio band and compresses a voice or a sound before transmitting the voice or the sound through a relay line. Nevertheless, this compression & decompression algorithm is designed specially for human voices.
For this reason, even if compression and decompression are carried out on a human voice, the human voice does not deteriorate much and the quality of the voice does not become poor. If compression and decompression are carried out on a signal other than a human voice by adopting the above algorithm, however, the signal will deteriorate substantially. Thus, when a fax signal passes through an audio compression/decompression functional unit, the signal deteriorates substantially. When the fax signal reaches a fax terminal on the receiver side, the signal has become a signal that cannot be recognized by the fax terminal as a fax signal. Thus, since a fax signal must not be passed through an audio compression/decompression functional unit, an audio-line processing unit is provided with a fax termination switching device in addition to the audio compression/decompression functional unit. If the input signal is an audio signal, the input signal is supplied to the audio compression/decompression functional unit. If the input signal is a fax signal, on the other hand, the input signal is supplied to the fax termination switching device instead of the audio compression/decompression functional unit. In this way, a fax signal can be received correctly without carrying out processing designed for a human voice.
In addition, when an analog fax signal passes through a digital network, a delay caused by the network may become a problem. In the case of fax communication through an analog telephone line, the fax machine on the transmitting side is perfectly connected to the fax machine on the receiving side by a telephone line. Thus, a fax communication can be carried out without causing a problem in particular. If the fax signal must pass through a digital network between telephone lines, however, a delay is introduced. Such a delay is caused by data processing such as analog-to-digital conversion, digital-to-analog conversion, packetization and depacketization. In a fax terminal, a delay is not assumed to exist. Thus, when an expected signal is not received within a fixed period of time, the fax terminal enters a time-out state, making a fax communication impossible in some cases. In order to solve this problem, a fax termination unit mounted on an audio communication line processing unit employed in a communication apparatus connected to the fax terminal pretends to function as a partner fax terminal. In this way, a time-out state can be avoided. The fax termination units exchange data through the digital network. Fax data is transmitted to the fax termination unit on the communication-partner side and then forwarded to a fax terminal connected to the fax termination unit. Thus, an end-to-end communication can be carried out between fax terminals.
In order to carry out a fax communication through an audio communication line of an ATM communication network, an IP communication network or another digital communication network by using the VTOA, the VOIP, the VOFR or the like, it is necessary to provide a fax termination unit in the communication apparatus. In the case of the VOIP, it is necessary to provide a mechanism conforming to T.38 specifications if a fax termination unit is not used. In a normal case, an analog fax communication through a PSTN line is generally carried out at a speed of 14,400 bps in accordance with a procedure conforming to T.30 specifications. This fax communication is referred to as an old G3 fax communication. In the year of 1996, Annex F is added to the T.30 specifications to allow a communication to be carried out at a high speed of typically 33,600 bps by using a modem in accordance with V.8, V.34 or the like. In this way, a technique of carrying out a fax communication at a higher speed is set as a standard. The number of fax terminals conforming to this standard increases. Fax communication conforming to this standard is called super G3 fax. To be more specific, the super G3 fax communication is fax communication carried out at a speed of close to 33,600 kbps using V.34 after a preceding negotiation between terminals using a V.8-recommendation signal having a speed of 300 bps. With the advent of the super G3 fax terminal capable of carrying out a fax communication at a high speed, it is possible to replace the G3 fax terminal becoming obsolete, as far as fax communication is concerned, with such a super G3 fax terminal without changing the configuration and operation conditions of the conventional communication network.
If the G3 fax terminal becoming obsolete, as far as fax communication is concerned, is replaced by such a super G3 fax terminal, however, the following problems rise. When a super G3 fax terminal with a broad band such as 33,600 kbps is connected to a communication apparatus employing the conventional G3 fax termination unit, the G3 fax termination unit is not capable of understanding the V.8 recommendation in a first negotiation with the super G3 fax terminal. Thus, the G3 fax termination unit does not recognize a negotiation signal as a fax signal, remaining in an inoperative state. As a result, the communication apparatus is not switched to the G3 fax termination unit. Much like an ordinary audio signal, a negotiation signal output by a super G3 fax terminal is transmitted through an audio compression/decompression functional unit. In a fax communication between super G3 fax terminals, a negotiation using a V.8-recommendation signal is carried out at a low rate of 300 bps.
As described above, an audio compression/decompression functional unit carries out processing specially provided for voices or sounds so that, when a fax signal passes through the audio compression/decompression functional unit, a fax terminal is not capable of carrying out a communication. Even if a signal having a low rate of 300 bps prescribed by the V.8 recommendation passes through the audio compression/decompression functional unit and is subjected to the processing specially provided for voices or sounds, however, a fax terminal on the communication-partner side will be capable of recognizing the signal as a V.8 signal in some cases. This is because the signal is transmitted at a low rate of 300 bps. Thus, a V.8-recommendation signal used as a negotiation signal of a super G3 fax terminal is transmitted successfully to a fax terminal on the communication-partner side by way of an audio compression/decompression functional unit so that a negotiation between fax terminals each serving as a super G3 fax terminal can be completed normally. Thus, the fax terminals each serving as a super G3 fax terminal are capable of starting a communication between them. Thereafter, the super G3 fax terminals make an attempt to establish a communication at 33,600 bps in accordance with the V.34 recommendation set for the super G3 fax communication. At the speed of 33,600 bps, however, a communication cannot be continued normally due to an overflow over a band allocated to compression of a signal transmitted at a speed of 8 kbps, 16 kbps or the like. In addition, since the fax termination unit is not capable of understanding a super G3 fax signal, the communication apparatus serving as the communication partner is not switched to the fax termination unit. Thus, since the fax signal passes through the audio compression/decompression functional unit, processing customized for voices or sounds is additionally carried out on the fax signal. As a result, the fax communication between the super G3 fax terminals is certainly lost.
In addition, in a communication according to the V.34-recommendation procedure, an ordinary fax termination unit is capable of understanding a transmitted signal but there is no means for handing over the communication to a G3 Fax-communication procedure capable of switching the communication to a fax communication in accordance with the recommendation. Thus, there is no method except a technique for cutting off the fax communication. For this reason, if a super G3 fax terminal is connected to a communication apparatus, the super G3 fax terminal needs to be set to carry out G3 fax communications instead of super G3 fax communications. Once the calling super G3 fax terminal merely receives an ANSam signal transmitted at the beginning of a negotiation according to the V.8 recommendation for super G3 fax terminals, however, the communication completely transits to the super G3 fax procedure and there is no means for returning the communication to the ordinary G3 fax communication procedure. Thus, at the time of a first negotiation between super G3 fax terminals, it is already impossible to carry out an ordinary G3 fax communication. It should be noted that an ANSam signal is a signal with a frequency of 2,100 Hz and phase inversions at intervals of 450 ms. The signal completes amplitude modulation at a frequency of 15 Hz.
In addition, in the case of a mechanism for implementing real-time fax communication in a VOIP network, that is, a mechanism prescribed in the T.38 specifications wherein an audio signal and a fax signal are accommodated in a packet with a fixed frame length and transmitted by using a fixed band, in an attempt made to carry out a super G3 fax communication by connecting a super G3 fax terminal, the super G3 fax communication will end in a failure since, at the present time, T.38 is not compatible with the super G3 fax communication.